Semiconductor device including MISFET having internal stress film

ABSTRACT

A semiconductor device includes a first-type internal stress film formed of a silicon oxide film over source/drain regions of an nMISFET and a second-type internal stress film formed of a TEOS film over source/drain regions of a pMISFET. In a channel region of the nMISFET, a tensile stress is generated in the direction of movement of electrons due to the first-type internal stress film, so that the mobility of electrons is increased. In a channel region of the pMISFET, a compressive stress is generated in the direction of movement of holes due to the second-type internal stress film, so that the mobility of holes is increased.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No. 11/730,988, filed Apr. 5, 2007, now U.S. Pat. No. 7,417,289, which is a Continuation of U.S. application Ser. No. 10/859,219, filed Jun. 3, 2004, now U.S. Pat. No. 7,205,615, and claiming priority of Japanese Application No. 2003-170335, filed Jun. 16, 2003, the entire contents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device including an MISFET and a method for fabricating the same, and more particularly relates to a measure for increasing the mobility of carriers.

When a stress is generated in a semiconductor crystal layer, a crystal-lattice constant varies and a band structure is changed, so that the mobility of carriers is changed. This phenomenon has been known as the “piezo resistivity effect”. Whether the carrier mobility is increased or reduced differs depending on the plane direction of a substrate, the direction in which carriers move, and whether the stress is a tensile stress or a compressive stress. For example, in an Si (100) substrate, i.e., a silicon substrate of which the principal surface is the {100} plane, assume that carriers move in the [011] direction. When carriers are electrons, with a tensile stress generated in the direction in which electrons in a channel region move, the mobility of the carriers is increased. On the other hand, when carriers are holes, with a compressive stress generated in the direction in which holes in a channel region move, the mobility of the carriers is increased. The increase rate of carrier mobility is proportional to the size of a stress.

In this connection, conventionally, there have been proposals for increasing carrier mobility by applying a stress to a semiconductor crystal layer to increase the operation speed of transistors and the like. For example, in Reference 1, an entire semiconductor substrate is bent using an external device, thereby generating a stress in an active region of a transistor.

SUMMARY OF THE INVENTION

However, in the above-described known structure, an external device is needed in addition to a semiconductor substrate and a stress can be generated only in the same direction in an entire region of the semiconductor substrate in which active regions of a transistor and the like are provided and which is located in the principal surface side. For example, when an Si (100) substrate is used, neither the mobility of electrons nor the mobility of holes can be increased.

It is therefore an object of the present invention to provide, by generating a stress which increases the mobility of carriers in a semiconductor layer without using an external device, a semiconductor device including a pMISFET and an nMISFET of which respective operation speeds are increased and a method for fabricating the same.

A semiconductor device according to the present invention includes an internal stress film for generating a stress in a gate length direction in a channel region of an active region in which a MISFET is formed.

Thus, the mobility of carriers in the MISFET can be increased by using the piezo resistivity effect.

The internal stress film is capable of covering one or both of source/drain regions. In an nMISFET, the internal stress film generates a tensile stress substantially in the parallel direction to a gate length direction in a channel region (i.e., the direction of movement of electrons). In a pMISFET, the internal stress film generates a compressive stress substantially in the parallel direction to a gate length direction in a channel region (i.e., the direction of movement of holes).

Covering both side surfaces or both side and upper surfaces of a gate electrode, the internal stress film can generate a stress in the longitudinal direction of the channel region through the gate electrode, thereby increasing the mobility of carriers.

Moreover, covering a side surface of the gate electrode and an upper surface of the semiconductor substrate in two regions of the substrate sandwiching part of the gate electrode, whether the MISFET is an nMISFET or a pMISFET, the internal stress film can generate a tensile stress substantially in the parallel direction to the gate width direction of the MISFET, thereby increasing the mobility of carriers.

A first method for fabricating a semiconductor device according to the present invention is a method in which an nMISFET and a pMISFET are formed in first and second active regions of a semiconductor substrate, respectively, and then first and second internal stress films which cover source/drain regions of the nMISFET and source/drain regions of the pMISFET, respectively, and generate a tensile stress and a compressive stress, respectively, substantially in the parallel directions to respective gate length directions of the channel regions are formed.

According to this method, a CMOS device of which the operation speed is increased can be obtained.

A second method for fabricating a semiconductor device according to the present invention is a method in which an internal stress film is formed first, a groove is formed in the internal stress film, a gate insulating film and a buried gate electrode are formed in the groove, and then the internal stress film is removed.

According to this method, a stress which increases the mobility of carriers in the channel region can be generated using a remaining stress in the gate insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention.

FIG. 2A through 2C are cross-sectional views illustrating first half of respective steps for fabricating the semiconductor device of the first embodiment.

FIG. 3A through 3C are cross-sectional views illustrating latter half of respective steps for fabricating the semiconductor device of the first embodiment.

FIGS. 4A through 4C are cross-sectional views illustrating first, second and third modified examples of the first embodiment.

FIGS. 5A through 5D are cross-sectional views illustrating respective steps for fabricating a semiconductor device according to the first modified example of the first embodiment.

FIGS. 6A through 6C are cross-sectional views illustrating respective steps for fabricating a semiconductor device according to the third modified example of the first embodiment.

FIGS. 7A through 7D are cross-sectional views illustrating first half of respective steps for fabricating a semiconductor device according to a second embodiment of the present invention.

FIGS. 8A through 8D are cross-sectional views illustrating latter half of respective steps for fabricating the semiconductor device of the second embodiment.

FIGS. 9A and 9B are a plane view of an MISFET of a semiconductor device according to a third embodiment of the present invention and a cross-sectional view illustrating a cross-sectional structure taken along the line IX-IX (a cross section in the gate width direction), respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention. As shown in FIG. 1, a surface region of a semiconductor substrate 1, i.e., an Si (100) substrate is divided into a plurality of active regions 1 a and 1 b by an isolation region 2. The semiconductor device includes an nMISFET formation region Rn which includes the active region 1 a and in which an nMISFET is to be formed and a pMISFET formation region Rp which includes the active region 1 b and in which a pMISFET is to be formed.

The nMISFET includes n-type source/drain regions 3 a and 4 a each of which includes an n-type lightly doped impurity region, an n-type heavily doped impurity region and a silicide layer such as a CoSi₂ layer, a gate insulating film 5 formed on the active region 1 a and made of a silicon oxide film, a silicon oxynitride film or the like, a gate electrode 6 a formed on the gate insulating film 5 and made of polysilicon, aluminum or the like, and a sidewall 7 covering a side surface of the gate electrode 6 a and made of an insulating film. Part of the active region 1 a located under the gate electrode 6 a is a channel region 1 x in which electrons move (travel) when the nMISFET is in an operation state.

The pMISFET includes p-type source/drain regions 3 b and 4 b each of which includes a p-type lightly doped impurity region, a p-type heavily doped impurity region and a silicide layer such as a CoSi₂ layer, a gate insulating film 5 formed on the active region 1 b and made of a silicon oxide film, a silicon oxynitride film or the like, a gate electrode 6 b formed on the gate insulating film 5 and made of polysilicon, aluminum or the like, and a sidewall 7 covering a side surface of the gate electrode 6 b and made of an insulating film. Part of the active region 1 b located under the gate electrode 6 b is a channel region 1 y in which holes move (travel) when the pMISFET is in an operation state.

Moreover, provided are a first-type internal stress film 8 a formed on the source/drain regions 3 a and 4 a of the nMISFET, made of a silicon nitride film or the like, and having a thickness of about 20 nm, a second-type internal stress film 8 b formed on the source/drain regions 3 b and 4 b of the pMISFET, made of a TEOS film or the like, and having a thickness of about 20 nm, an interlevel insulating film 9 covering the nMISFET and pMISFET and having a surface flattened, a lead electrode 10 formed on the interlevel insulating film 9, and a contact 11 connecting each of the source/drain regions 3 a, 3 b, 4 a and 4 b with the lead electrode 10 through the interlevel insulating film 9.

Herein, an “internal stress film” is a film characterized in that where the internal stress film is directly in contact with some other member or faces some other member with a thin film interposed therebetween, a stress is generated in the film itself. As for stress, there are tensile stress and compressive stress. In this embodiment and other embodiments, an internal stress film in which a tensile stress is generated substantially in the parallel direction to the direction in which carriers move (i.e., the gate length direction) in a channel region of an MISFET is referred to as a “first-type internal stress film” and an internal stress film in which a compressive stress is generated substantially in the parallel direction to the direction in which carriers move (the gate length direction) in a channel region of an MISFET is referred to as a “second-type internal stress film”.

Herein, the semiconductor substrate 1 is an Si substrate of which the principal surface is the {100} plane and is referred to as an Si (100) substrate for convenience. However, the {100} plane is a general name for the (±100) plane, the (0±10) plane and the (00±1) plane, and therefore, even a plane which is not exactly the {100} plane and is tilted from the {100} plane by a less angle than 10 degree is considered to be substantially the {100} plane. Moreover, in this embodiment, the direction in which electrons move in the nMISFET and the direction in which holes move in the pMISFET (i.e., the gate length direction of each MISFET) is the [011] direction. However, in this embodiment, the “[011] direction on the principal surface of an Si (100) substrate” includes equivalent directions to the [011] direction, such as the [01-1] direction, the [0-11] direction, and the [0-1-1] direction, i.e., directions within the range of the <011> direction. That is, even a direction which is not exactly the [011] direction and tilted from the <011> direction by a less angle than 10 degree is considered to be substantially the [011] direction.

According to this embodiment, the following effects can be obtained.

In the nMISFET, when the first-type internal stress film 8 a is brought into a direct contact with a semiconductor layer or made to face a semiconductor layer with a thin film interposed therebetween, a stress for compressing the first-type internal stress film itself, i.e., a compressive stress is generated in the first-type internal stress film 8 a. As a result, by the first-type internal stress film 8 a, the semiconductor layer adjacent to the first-type internal stress film 8 a can be stretched in the vertical direction to a boundary surface. Specifically, the first-type internal stress film 8 a applies a compressive stress to the source region 3 a and the drain region 4 a in the active region 1 a of the nMISFET in the parallel direction to the principal surface. As a result, a tensile stress is applied to a region of the substrate located between the source region 3 a and the drain region 4 a, i.e., the channel region 1 x in the gate length direction (the direction in which electrons move when the nMISFET is in an operation state). Then, with this tensile stress, electrons are influenced by the piezo resistivity effect, so that the mobility of electrons is increased. Herein, “substantially in the parallel direction” also means in a direction tilted by an angle of less than 10 degree from the direction in which electrons move.

For example, assume that the substrate 1 is an Si (100) substrate and the direction in which electrons move is the [011] direction. When the internal stress of the first-type internal stress film 8 a adjacent to the semiconductor layer is a general level for a silicon nitride film, i.e., 1.5 GPa, the thickness of the first-type internal stress film 8 a is 20 nm, a space between respective parts of the source and drain regions 3 a and 4 a being in contact with the first-type internal stress film 8 a, i.e., the length of the channel region 1 x, is 0.2 μm, a tensile stress in the gate length direction generated at a depth of 10 nm from the surface of the substrate is 0.3 GPa (J. Appl. Phys., vol. 38-7, p. 2913, 1967) and the improvement rate of the mobility of electrons is +10% (Phys. Rev., vol. 94, p. 42, 1954). To obtain a larger change in the mobility than this, the tensile stress of a semiconductor can be increased. Thus, a film having a large internal stress can be used as the first-type internal stress film 8 a, the thickness of the first-type internal stress film 8 a can be increased, or the space between the parts of the source and drain regions 3 a and 4 a being in contact with the first-type internal stress film 8 a, i.e., the length of the channel region 1 x, can be reduced for a larger change in the mobility. For example, when the thickness of the first-type internal stress film 8 a is doubled, the space between the parts of the source and drain regions 3 a and 4 a being in contact with the first-type internal stress film 8 a, i.e., the length of the channel region 1 x is reduced to half, the improvement rate of the mobility of electrons is +40%. As another way to obtain a large mobility, the direction in which electrons move is changed from the [011] direction to the [010] direction to change the improvement rate of the mobility of electrons with respect to a tensile stress. As a result, with the same tensile stress, the improvement rate of the mobility becomes about 3.5 times large. Although the source and drain regions 3 a and 4 a receive compressive stresses by the first-type internal stress film 8 a, influence of the piezo resistivity effect is small because a low-resistant heavily doped semiconductor device and a silicide film are used. Moreover, influence of the internal stress of the interlevel insulating film 9 on the channel region can be neglected. This is because with the substrate covered by the interlevel insulating film 9, internal stresses in the interlevel insulating film 9 are cancelled off with each other, so that the function of applying stress to the active regions 1 a and 1 b is small.

In the pMISFET, when the second-type internal stress film 8 b is brought into a direct contact with the semiconductor layer or made to face a semiconductor layer with a thin film interposed therebetween, a stress for stretching the second-type internal stress film itself, i.e., a tensile stress is generated in the second-type internal stress film 8 b. As a result, by the second-type internal stress film 8 b, the semiconductor layer adjacent to the second-type internal stress film 8 b is compressed in the vertical direction to a boundary surface. Specifically, the second-type internal stress film 8 b applies a tensile stress to the source region 3 b and the drain region 4 b in the active region 1 b of the pMISFET in the parallel direction to the principal surface. As a result, a compressive stress is applied to a region of the substrate located between the source region 3 b and the drain region 4 b, i.e., the channel region 1 y substantially in the parallel direction to the gate length direction (the direction in which holes move when the pMISFET is in an operation state). Then, with this compressive stress, holes are influenced by the piezo resistivity effect, so that the mobility of holes is increased. Herein, “substantially in the parallel direction” also means in a direction tilted by an angle of less than 10 degree from the direction in which electrons move.

Note that, instead of the internal stress films 8 a and 8 b, the semiconductor film itself in which the source and drain regions 3 a, 4 a, 3 b and 4 b are formed may be a film having an internal stress, for example, an uppermost semiconductor layer in an SOI substrate.

Furthermore, each of the internal stress films 8 a and 8 b does not have to be a single layer but may include multiple layers, as long as each of the internal stress films 8 a and 8 b can apply a stress to the substrate as a whole.

Moreover, in this embodiment, an Si (100) substrate is used. However, even if an Si (111) substrate is used, with the direction in which electrons move set to be the [001] direction, the mobility of electrons is increased by a tensile stress. In general, in any substrate plane directions, there is a direction of movement of electrons or holes, which allows increase in the mobility of electrons or holes according to the direction of a stress.

In this embodiment, the internal stress films 8 a and 8 b exist on the source/drain regions 3 a and 4 a and the source/drain regions 3 b and 4 b, respectively. However, even when the internal stress film 8 a exists only on one of the source/drain regions 3 a and 4 a and the internal stress film 8 b exists only on one of the source/drain regions 3 b and 4 b, the effect of increasing the mobility of carriers can be obtained. In this case, the improvement rate of the mobility is reduced to half. In each of the following embodiments, when an internal stress film exits only on one of source/drain regions, the improvement rate of the mobility is reduced to half, compared to the case where internal stress films exist on source/drain regions, but the mobility is increased.

FIGS. 2A through 2C and FIGS. 3A through 3C are cross-sectional views illustrating respective steps for fabricating a semiconductor device according to the first embodiment of the present invention.

First, in the process step of FIG. 2A, a trench and a buried oxide film are formed in part of a semiconductor substrate 1, i.e., an Si (100) substrate, thereby forming an isolation region 2 for dividing the substrate into active regions 1 a, 1 b and so on. Thereafter, after a gate insulating film 5 has been formed by thermal oxidation of respective surfaces of the active regions 1 a and 1 b and a polysilicon film for forming gate electrodes has been deposited, the polysilicon film and the gate insulating film 5 are etched by patterning using lithography and anisotropic dry etching, thereby forming gate electrodes 6 a and 6 b. The gate length direction of each of the gate electrodes 6 a and 6 b is the [011] direction. Next, using the gate electrode 6 a of the nMISFET as a mask, ion implantation of an n-type impurity (e.g., arsenic) at a low concentration is performed to an nMISFET formation region Rn at an injection energy of 10 keV and a dose of 1×10¹³/cm², and using the gate electrode 6 b of the pMISFET as a mask, ion implantation of a p-type impurity (e.g., boron) at a low concentration is performed to a pMISFET formation region Rp at an injection energy of 2 keV and a dose of 1×10¹⁵/cm². Thereafter, an insulating film which is for forming a sidewall and has a thickness of about 50 nm is deposited on the substrate and then a sidewall 7 is formed on side surfaces of the gate electrodes 6 a and 6 b by etch back. Next, using the gate electrode 6 a of the nMISFET and the sidewall 7 as masks, ion implantation of an n-type impurity (e.g., arsenic) at a high concentration is performed to the nMISFET formation region Rn at an injection energy of 20 keV and a dose of 1×10¹⁴/cm², and ion implantation of a p-type impurity (e.g., boron) at a high concentration is performed to the pMISFET formation region Rp at an injection energy of 5 keV and a dose of 1×10¹⁶/cm². Thereafter, thermal treatment (RTA) for activating impurities is performed. By the above-described processing, source/drain regions 3 a and 4 a including an n-type lightly doped impurity region and an n-type heavily doped impurity region are formed in the nMISFET formation region Rn and source/drain regions 3 b and 4 b including a p-type lightly doped impurity region and a p-type heavily doped impurity region are formed in the pMISFET formation region Rp.

Next, in the process step of FIG. 2B, a silicon nitride film 8 x is formed on the substrate so that the silicon nitride film 8 x has a relatively large thickness and a surface thereof is flatted. At this point of time, the silicon nitride film 8 x covers respective upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter, a resist film 12 is formed on the silicon nitride film 8 x by lithography and the silicon nitride film 8 x is patterned using the resist film 12 as a mask so that the silicon nitride film 8 x is left only on the nMISFET formation region Rn.

Next, in the process step of FIG. 2C, after the resist film 12 has been removed, the silicon nitride film 8 x is etched back, part of the silicon nitride film 8 x located on the gate electrode 6 a is removed and the thickness of the silicon nitride film 8 x is further reduced. Thus, a first-type internal stress film 8 a is formed. That is, the first-type internal stress film 8 a does not exist on the gate electrode 6 a of the nMISFET but exits only on the source/drain regions 3 a and 4 a.

Next, in the process step of FIG. 3A, a TEOS film 8 y is formed on the substrate so that the TEOS film 8 y has a relatively large thickness and a surface thereof is flatted. At this point of time, the TEOS film 8 y covers respective upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter, a resist film (not shown) is formed on the TEOS film 8 y by lithography and the TEOS film 8 y is patterned using the resist film as a mask so that the TEOS film 8 y is left only on the pMISFET formation region Rp.

Next, in the process step of FIG. 3B, after the resist film has been removed, the TEOS film 8 y is etched back, parts of the TEOS film 8 y located on the gate electrodes 6 a and 6 b are removed and the thickness of the TEOS film 8 y is further reduced. Thus, a second-type internal stress film 8 b having substantially the same thickness as that of the first-type internal stress film 8 a is formed. That is, the second-type internal stress film 8 b does not exist on the gate electrode 6 b of the pMISFET and the first-type internal stress film 8 a but exists only on the source/drain regions 3 b and 4 b.

By the above-described process steps, the internal stress films 8 a and 8 b for applying stresses in opposite directions to each other are formed on the source/drain regions 3 a and 4 a of the nMISFET and the source/drain regions 3 b and 4 b of the pMISFET, respectively.

Next, in the process step of FIG. 3C, on the substrate, an interlevel insulating film 9 is formed and then contact holes are formed so as to pass through the interlevel insulating film 9 and reach the source/drain regions 3 a and 4 a of the nMISFET by lithography and dry etching, the source/drain regions 3 b and 4 b, and the gate electrodes 6 a and 6 b, respectively. Thereafter, each of the contact holes is filled with metal (e.g., tungsten), thereby forming contact plugs 11. Furthermore, a metal film such as an aluminum alloy film is deposited on the interlevel insulating film 9 and then the metal film is patterned, thereby forming a lead electrode 10 connected to each of the contact plugs 11. Thus, the respective source/drain regions 3 a, 4 a, 3 b and 4 b of the MISFETs and the gate electrodes 6 a and 6 b are made to be electrically connectable from the outside.

In the fabrication method of this embodiment, either one of the two types of internal stress films 8 a and 8 b may be formed first. And the internal stress films 8 a and 8 b may overlap with each other over the isolation region 2 and the source/drain regions 3 a, 4 a, 3 b and 4 b.

First Modified Example of First Embodiment

FIGS. 4A through 4C are cross-sectional views illustrating first through third modified examples of the first embodiment.

A semiconductor device according to a first modified example shown in FIG. 4A has a structure in which the sidewall 7 of the first embodiment is omitted. Moreover, each of the source/drain regions 3 a, 4 a, 3 b and 4 b does not include a lightly doped impurity region and includes only a heavily doped impurity region. Other part has the same structure as that of the semiconductor device of the first embodiment. In this modified example, no sidewall exists in forming an internal stress film, so that a space between respective parts of the source/drain regions 3 a and 4 a being in contact with the first-type internal stress film 8 a is small. Thus, a stress applied to each of the channel regions 1 x and 1 y is increased, so that the effect of improving the carrier mobility becomes larger than that of the first embodiment.

A semiconductor device according to a second modified example shown in FIG. 4B has a structure in which instead of the sidewall 7 of the first embodiment, which is made of a silicon oxide film, the first-type internal stress film 8 a made of a silicon nitride film covers a side surface of the gate electrode 6 a of the nMISFET and the second-type internal stress film 8 b made of a TEOS film covers a side surface of the gate electrode 6 b of the pMISFET. Moreover, each of the source/drain regions 3 a, 4 a, 3 b and 4 b does not include a lightly doped impurity region and includes only a heavily doped impurity region. Other part has the same structure as that of the semiconductor device of the first embodiment.

In this modified example, in addition to the effect of the first modified example, the following effect can be obtained. In the nMISFET, the first-type internal stress film 8 a and the gate electrode 6 a are in contact with each other substantially at the entire side surface of the gate electrode 6 a, so that the gate electrode 6 a is compressed downwardly by the first-type internal stress film 8 a. With the gate electrode 6 a compressed downwardly, then, in the channel region 1 x, a compressive stress is generated in the vertical direction to the principal surface and the mobility of electrodes in the nMISFET is further improved.

Moreover, in the pMISFET, the second-type internal stress film 8 b and the gate electrode 6 b are in contact with each other substantially at the entire side surface of the gate electrode 6 b, so that the gate electrode 6 b is stretched upwardly by the second-type internal stress film 8 b. With the gate electrode 6 b stretched upwardly, then, in the channel region 1 y, a tensile stress is generated in the vertical direction to the principal surface and the mobility of holes in the pMISFET is further improved.

Therefore, in this structure, the improvement rate of the mobility is further increased, compared to the first modified example.

A semiconductor device according to a third modified example shown in FIG. 4C has a structure in which instead of the sidewall 7 of the first embodiment, which is made of a silicon oxide film, the first-type internal stress film 8 a made of a silicon nitride film covers side and upper surfaces of the gate electrode 6 a of the nMISFET and the second-type internal stress film 8 b made of a TEOS film covers side and upper surfaces of the gate electrode 6 b of the pMISFET. Moreover, each of the source/drain regions 3 a, 4 a, 3 b and 4 b does not include a lightly doped impurity region and includes only a heavily doped impurity region. Other part has the same structure as that of the semiconductor device of the first embodiment.

In this modified example, in addition to the effect of the second modified example, the following effect can be obtained. In the nMISFET, the first-type internal stress film 8 a and the gate electrode 6 a are in contact with each other substantially at the entire side and upper surfaces of the gate electrode 6 a, so that the effect of compressing the gate electrode 6 a downwardly by the first-type internal stress film 8 a becomes larger. With the gate electrode 6 a compressed downwardly more strongly, then, in the channel region 1 x, a compressive stress is generated in the vertical direction to the principal surface and the mobility of electrons in the nMISFET is further improved.

Moreover, in the pMISFET, the second-type internal stress film 8 b and the gate electrode 6 b are in contact with each other substantially at the entire side and upper surfaces, so that the stress of stretching the gate electrode 6 b upwardly by the second-type internal stress film 8 b becomes larger. With the gate electrode 6 b stretched upwardly more strongly, then, in the channel region 1 y, a tensile stress is generated in the vertical direction to the principal surface and the mobility of holes in the pMISFET is further more improved.

Therefore, in this structure, the improvement rate of the mobility is further increased, compared to the second modified example.

In each of the first through third modified examples, only the heavily doped impurity regions are provided as the source/drain regions. However, the source/drain regions may be source/drain regions including a lightly doped impurity region and a heavily doped impurity region. In this case, as shown in FIG. 2A, a lightly doped impurity region and a heavily doped impurity region are formed using a gate electrode and a sidewall, and then, after the sidewall has been removed, an internal stress film is formed in the manner shown in FIGS. 4A through 4C. Thus, source/drain regions including a lightly doped impurity region and a heavily doped impurity region can be formed.

Fabrication Method According to First Modified Example

FIGS. 5A through 5D are cross-sectional views illustrating respective steps for fabricating a semiconductor device according to the first modified example of the first embodiment.

First, before the process step of FIG. 5A, the same process step as that of FIG. 2A in the first embodiment is performed to form an isolation region 2 for dividing a substrate into active regions 1 a, 1 b and so on, a gate insulating film 5, gate electrodes 6 a and 6 b, source/drain regions 3 a and 4 a each including only an n-type heavily doped impurity region and source/drain regions 3 b and 4 b each including only a p-type heavily doped impurity region. In this case, each of the source/drain regions 3 a, 4 a, 3 b and 4 b includes only a heavily doped impurity region. However, the source/drain regions 3 a, 4 a, 3 b and 4 b may be source/drain regions each including a lightly doped impurity region and a heavily doped impurity region. In that case, by the process step of FIG. 2A, after a heavily doped impurity region has been formed using the sidewall as a mask, the sidewall is removed. Thus, source/drain regions each including a lightly doped impurity region and a heavily doped impurity region can be formed.

Next, in the process step of FIG. 5A, a silicon nitride film 8 x is formed on the substrate so that the silicon nitride film 8 x has a relatively large thickness and a surface thereof is flatted. At this point of time, the silicon nitride film 8 x covers respective upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter, a resist film 12 is formed on the silicon nitride film 8 x by lithography and the silicon nitride film 8 x is patterned using the resist film 12 as a mask so that the silicon nitride film 8 x is left on the nMISFET formation region Rn.

Next, in the process step of FIG. 5B, after the resist film 12 has been removed, the silicon nitride film 8 x is etched back, part of the silicon nitride film 8 x located on the gate electrode 6 a is removed and the thickness of the silicon nitride film 8 x is further reduced. Thus, a first-type internal stress film 8 a having a thickness of about 20 nm is formed. That is, the first-type internal stress film 8 a does not exist on the gate electrode 6 a of the nMISFET but exists only on the source/drain regions 3 a and 4 a.

Next, in the process step of FIG. 5C, a TEOS film 8 y is formed on the substrate so that the TEOS film 8 y has a relatively large thickness and a surface thereof is flatted. At this point of time, the TEOS film 8 y covers respective upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs.

Next, in the process step of FIG. 5D, the TEOS film 8 y is etched back, parts of the TEOS film 8 y located on the first-type internal stress film 8 a and the gate electrode 6 b of the pMISFET are removed and the thickness of the TEOS film 8 y is further reduced. Thus, a second-type internal stress film 8 b having substantially the same thickness as that of the first-type internal stress film 8 a is formed.

By the above-described process steps, the internal stress films 8 a and 8 b for applying stresses in opposite directions to each other are formed on the source/drain regions 3 a and 4 a of the nMISFET and on the source/drain regions 3 b and 4 b of the pMISFET, respectively.

Although illustration of the subsequent process step will be omitted, the same process step as that of FIG. 3C is performed to form an interlevel insulating film, contact holes reaching the source/drain regions 3 a and 4 a of the nMISFET, the source/drain regions 3 b and 4 b of the pMISFET, and the gate electrodes 6 a and 6 b, respectively, through the insulating film 9, and contact plugs 11. Furthermore, lead electrodes are formed on the interlevel insulating film, so that the source/drain regions 3 a, 4 a, 3 b and 4 b and the gate electrodes 6 a and 6 b of the MISFETs becomes electrically connectable from the outside.

According to this embodiment, by forming the first-type internal stress film 8 a and the second-type internal stress film 8 b, the structure shown in FIG. 4A can be obtained. Thus, no sidewall exists, so that a space between respective parts of the source/drain regions 3 a and 4 a which are in contact with the first-type internal stress film 8 a is small. Accordingly, a stress applied to each of the channel regions 1 x and 1 y is increased, so that the effect of improving the carrier mobility becomes larger than that of the first embodiment. The distance between the active region 1 a and the first-type internal stress film 8 a is reduced, so that a tensile stress generated in the active region is increased.

Fabrication Method According to Third Modified Example

FIGS. 6A through 6C are cross-sectional views illustrating respective steps for fabricating a semiconductor device of the third modified example of the first embodiment.

First, before the process step of FIG. 6A, the same process step as that of FIG. 2A in the first embodiment is performed to form an isolation region 2 for dividing a substrate into active regions 1 a, 1 b and so on, a gate insulating film 5, gate electrodes 6 a and 6 b, source/drain regions 3 a and 4 a each including only an n-type heavily doped impurity region and source/drain regions 3 b and 4 b each including only a p-type heavily doped impurity region. In this case, each of the source/drain regions 3 a, 4 a, 3 b and 4 b includes only a heavily doped impurity region. However, the source/drain regions 3 a, 4 a, 3 b and 4 b may be source/drain regions each including a lightly doped impurity region and a heavily doped impurity region. In that case, by the process step of FIG. 2A, after a heavily doped impurity region has been formed using the sidewall as a mask, the sidewall is removed.

Next, in the process step of FIG. 6A, a silicon nitride film having a smaller thickness than those of the gate electrodes 6 a and 6 b, i.e., a thickness of about 20 nm is formed on the substrate. At this point of time, the silicon nitride film covers side and upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter, a resist film 12 is formed on the silicon nitride film by lithography and the silicon nitride film is patterned using the resist film 12 as a mask, thereby forming a compressive stress film 8 a made of the silicon nitride film only in an nMISFET formation region Rn.

Next, in the process step of FIG. 6B, after the resist film 12 has been removed, a TEOS film having a smaller thickness than those of the gate electrodes 6 a and 6 b, i.e., a thickness of about 20 nm is formed on the substrate. At this point of time, the TEOS film covers side and upper surfaces of the gate electrodes 6 a and 6 b of the MISFETs as well as part of the first-type internal stress film 8 a. Next, a resist film 13 is formed on the TEOS film by lithography and the TEOS film is patterned using the resist film 13 as a mask to remove part of the TEOS film located on the gate electrode 6 b of a pMISFET, thereby forming a second-type internal stress film 8 b which overlaps with the first-type internal stress film 8 a over the isolation region 2. At this point of time, ideally, it is more preferable that the first-type internal stress film 8 a and the second-type internal stress film 8 b do not overlap with each other. However, even though the first-type internal stress film 8 a and the second-type internal stress film 8 b overlap with each other over the isolation region 2 or the source/drain regions, a stress which the second-type internal stress film 8 b applied to the active region 1 a is small because there is a large distance therebetween.

By the above-described process steps, the internal stress films 8 a and 8 b for applying stresses in opposite directions to each other are formed on the source/drain regions 3 a and 4 a of the nMISFET and the source/drain regions 3 b and 4 b of the pMISFET, respectively.

Next, in the process step of FIG. 6C, the resist film 13 is removed and then the same process step as that of FIG. 3C in the first embodiment is performed to form an interlevel insulating film 9, contact holes reaching the source/drain regions 3 a and 4 a of the nMISFET, the source/drain regions 3 b and 4 b of the pMISFET, and the gate electrodes 6 a and 6 b, respectively, through the insulating film 9, and contact plugs 11. Furthermore, lead electrodes 10 are formed on the interlevel insulating film 9, so that the respective source/drain regions 3 a, 4 a, 3 b and 4 b and the gate electrodes 6 a and 6 b of the MISFETS can be electrically accessed from the outside.

The semiconductor device of this modified example has a structure in which the first-type internal stress film 8 a made of a silicon nitride film covers the side and supper surfaces of the gate electrode 6 a of the nMISFET and the second-type internal stress film 8 b made of a TEOS film covers the side and upper surfaces of the gate electrode 6 b of the pMISFET. Other part has the same structure as that of the semiconductor device of the first embodiment.

In this modified example, the following effect can be obtained. In the nMISFET, the first-type internal stress film 8 a and the gate electrode 6 a are in contact with each other substantially at the entire side and upper surfaces of the gate electrode 6 a, so that the gate electrode 6 a is strongly compressed downwardly by the first-type internal stress film 8 a. With the gate electrode 6 a compressed downwardly, then, in the channel region 1 x, a compressive stress is generated in the vertical direction to the principal surface. In this embodiment, the Si (100) substrate is used and the direction of movement of electrons is the [011] direction, so that the mobility of electrons in the nMISFET is further improved by this compressive stress.

Moreover, in the pMISFET, the second-type internal stress film 8 b and the gate electrode 6 b are in contact with each other substantially at the entire side and upper surfaces of the gate electrode 6 b, so that the gate electrode 6 b is strongly stretched upwardly by the second-type internal stress film 8 b. With the gate electrode 6 b stretched upwardly, then, in the channel region 1 y, a tensile stress is generated in the vertical direction to the principal surface. In this embodiment, the Si (100) substrate is used and the direction of movement of holes is the [011] direction, so that the mobility of holes in the pMISFET is further improved by this tensile stress.

Furthermore, in the fabrication method of this embodiment, in the process step of forming the first-type internal stress film 8 a of a silicon nitride film and the process step of forming the second-type internal stress film 8 b of a TEOS film, patterning is performed, instead of etch back. Therefore, the fabrication method is more simplified.

Second Embodiment

FIGS. 7A through 7D and FIGS. 8A through 8D are cross-sectional views illustrating respective steps for fabricating a semiconductor device according to a second embodiment of the present invention.

First, in the process step of FIG. 7A, a trench and a buried oxide film are formed in part of a semiconductor substrate 1, thereby forming an isolation region 2 for dividing the substrate into active regions 1 a, 1 b and so on.

Next, in the process step of FIG. 7B, a silicon nitride film is formed on the substrate so as to have a larger thickness than that of a gate electrode. At this point of time, a gate electrode of each MISFET is not formed and thus the silicon nitride film covers each of the active regions 1 a and 1 b and the isolation region 2. Thereafter, a resist film 12 is formed on the silicon nitride film by lithography and then the silicon nitride film is patterned using the resist film 12 as a mask, thereby forming a first-type internal stress film 8 a made of the silicon nitride film only in an nMISFET formation region Rn.

Next, in the process step of FIG. 7C, a TEOS film is formed on the substrate so as to have a larger thickness than that of a gate electrode. At this point of time, a gate electrode of each MISFET is not formed and thus the TEOS film covers part of the first-type internal stress film 8 a, the active region 1 b and the isolation region 2. Thereafter, a resist film 13 is formed on the TEOS film and the TEOS film is patterned using the resist film 13 as a mask, thereby forming a second-type internal stress film 8 b made of the TEOS film in a pMISFET formation region Rp. In this case, the first-type internal stress film 8 a and the second-type internal stress film 8 b overlap with each other over the isolation region 2.

Next, in the process step of FIG. 7D, a resist film 14 is formed on the first-type internal stress film 8 a and the second-type internal stress film 8 b by lithography so as to have openings each corresponding to a gate formation region of each MISFET and then the first-type internal stress film 8 a and the second-type internal stress film 8 b are etched using the resist film 14 as a mask, thereby forming grooves 6 x and 6 y each of which reaches an upper surface of the semiconductor substrate 1. In this case, a tensile stress in the lateral direction is generated in a channel region 1 x of the active region 1 a by the first-type internal stress film 8 a and a compressive stress is generated in a channel region 1 y of the active region 1 b.

Next, in the process step of FIG. 8A, after the resist film 14 has been removed, thermal treatment is performed to regions of the semiconductor substrate 1 exposed at the respective bottoms of the grooves 6 x and 6 y, thereby forming a gate insulating film 5. Next, a polysilicon film 6 is deposited in the grooves 6 x and 6 y and on the first-type internal stress films 8 a and the second-type internal stress film 8 b to a thickness with which the upper surface of the polysilicon film 6 becomes substantially flatted.

Next, in the process step of FIG. 8B, the polysilicon film 6 is etched back to remove parts of the polysilicon film 6 located over the first-type internal stress film 8 a and the second-type internal stress film 8 b and to fill each of the grooves 6 x and 6 y with the polysilicon film, thereby forming gate electrodes 6 a and 6 b. Furthermore, lithography and ion implantation are performed to dope the gate electrode 6 a of the nMISFET formation region Rn with an n-type impurity and the gate electrode 6 b of the pMISFET formation region Rp with a p-type impurity, thereby reducing the resistance of each of the gate electrodes 6 a and 6 b.

Next, in the process step of FIG. 8C, after the internal stress films 8 a and 8 b have been removed by selective etching, lithography and ion implantation using each of the gate electrodes 6 a and 6 b as a mask are performed so that n-type and p-type impurities are injected into the nMISFET formation region Rn and the pMISFET formation region Rp, respectively. Thus, source/drain regions 3 a and 4 a each including only an n-type heavily doped impurity region of the nMISFET and source/drain regions 3 b and 4 b each including only a p-type heavily doped impurity region of the pMISFET are formed. Stresses which have been applied to the channel regions 1 x and 1 y by the internal stress films 8 a and 8 b are reduced by removing the internal stress films 8 a and 8 b. However, the gate insulating film 6 is formed with stresses generated in the channel regions 1 x and 1 y, and therefore, the gate insulating film 5 holds stresses in the channel regions 1 x and 1 y, i.e., regions of the substrate located directly under the gate insulating film 5. For example, assume that an Si (100) substrate is used as the semiconductor substrate 1 and a silicon oxide film is used as the gate insulating film 5. When the internal stress films 8 a and 8 b are removed, each of stresses in the channel regions 1 x and 1 y is reduced to one fourth because the ratio between the respective Young's moduli of the silicon oxide film and the Si (100) substrate is 1:3. To keep each of a tensile stress and a compressive stress at a large level, an insulation material of which the Young's modulus is larger than that of the Si (100) substrate can be used as the gate insulating film 5. A silicon nitride film is an example of insulation materials with a larger Young's modulus than that of the Si (100) substrate. In this case, each of the source/drain regions 3 a, 4 a, 3 b and 4 b includes only a heavily doped impurity region. However, the source/drain regions 3 a, 4 a, 3 b and 4 b may be source/drain regions each including a lightly doped impurity region and a heavily doped impurity region. In that case, after a lightly doped impurity region has been formed by ion implantation at a low concentration using a gate electrode as a mask, a sidewall can be formed on a side surface of the gate electrode and then a heavily doped impurity region can be formed by ion implantation at a high concentration using the sidewall as a mask.

Next, in the process step of FIG. 8D, the same process step as that of FIG. 3C in the first embodiment is performed to form an interlevel insulating film 9, contact holes reaching the source/drain regions 3 a and 4 a of the nMISFET, the source/drain regions 3 b and 4 b of the pMISFET, and the gate electrodes 6 a and 6 b, respectively, through the insulating film 9, and contact plugs 11. Furthermore, lead electrodes 10 are formed on the interlevel insulating film 9, so that the respective source/drain regions 3 a, 4 a, 3 b and 4 b and the gate electrodes 6 a and 6 b of the MISFETs can be electrically accessed from the outside.

In this embodiment, compared to the first embodiment, the following advantage can be obtained. In the first embodiment, there is possibility that when stresses are generated in the channel regions 1 x and 1 y, stresses to be generated by the first-type internal stress film 8 a and the second-type internal stress film 8 b in the channel regions 1 x and 1 y might be interrupted by the gate electrodes 6 a and 6 b and then reduced because the gate electrodes 6 a and 6 b exist. In contrast, in this embodiment, when the grooves 6 x and 6 y are formed in the process step of FIG. 7D, the semiconductor substrate 1 is the only member which is in contact with the internal stress films 8 a and 8 b. Therefore, a stress applied to each of the channel regions 1 x and 1 y is increased.

Note that in this embodiment, a polysilicon film which is doped with an impurity is used for the gate electrodes 8 a and 8 b. However, if as a film for forming the gate electrodes 8 a and 8 b, a film which is conductive without being doped with an impurity (for example, a metal film such as a copper film, a tungsten film, a cobalt film and a nickel film) is used instead of a polysilicon film, the subsequent ion implantation is not necessary. Therefore, fabrication process steps can be simplified.

Moreover, in this embodiment, the internal stress films 8 a and 8 b are removed for the purpose of performing ion implantation for forming the source/drain regions 3 a, 4 a, 3 b and 4 b. However, if the thicknesses of the internal stress films 8 a and 8 b are small, impurity ions can be injected into the semiconductor substrate 1 through the internal stress films 8 a and 8 b. Therefore, the internal stress films 8 a and 8 b can be left without being removed.

Moreover, in the process step of FIG. 8A, exposed part of a surface portion of the semiconductor substrate is thermally oxidized to form a gate insulating film. However, a gate insulating film can be deposited by CVD or PVD. In that case, an insulating film (e.g., a silicon oxide film) for forming a gate insulating film is left on a side surface of each of the gate electrodes 6 a and 6 b. Even if this insulating film if left as a sidewall, no problem is caused.

Third Embodiment

FIGS. 9A and 9B are a plane view of an MISFET in a semiconductor device according to a third embodiment of the present invention and a cross-sectional view illustrating a cross-sectional structure taken along the line IX-IX (a cross section in the gate width direction), respectively. In this embodiment, a structure for generating a stress in the gate width direction of a MISFET, i.e., in the direction which is parallel to the principal surface of a semiconductor substrate and vertical to the direction of movement of carriers will be described.

As shown in FIGS. 9A and 9B, in an active region 17 of a semiconductor substrate 1, i.e., an Si (100) substrate, provided is a MISFET including a gate insulating film 5, a gate electrode 15 and source/drain regions 3 and 4. The MISFET may be either an nMISFET or a pMISFET. Moreover, at each of both ends of the gate electrode 15, first-type internal stress films 16 which spreads over part of side, end and upper surfaces of the gate electrode 15, the semiconductor substrate 1 and the isolation region 2 and is made of a silicon nitride film are provided.

In this embodiment, a compressive stress is generated by each of the first-type internal stress films 16 at each end of each of the source/drain regions 3 and 4 located directly under the first-type internal stress films 16. In regions of the source/drain regions 3 and 4 on which the first-type internal stress film 16 is not located, a tensile stress is generated in the gate width direction. Furthermore, with the gate electrode 15 having both ends thereof compressed by the first-type internal stress films 16, a tensile stress is generated in the gate width direction in part of the gate electrode 15 on which the first-type internal stress films 16 are not located. As a result, in the channel region 17 x, a stress due to the source/drain regions 3 and 4 to which a tensile stress is applied and a stress due to the channel region 17 x stretched by the gate electrode 15 to which a tensile stress is applied are generated, so that a large tensile stress is generated in the gate width direction. The direction of the tensile stress is the parallel direction to the principal surface of the semiconductor substrate 1 and also the vertical direction to the direction in which carriers move. Assume that an Si (100) substrate is used as the semiconductor substrate 1 and the gate length direction is set to be the [011] direction. Whether carriers running through the channel region 17 x are electrons or holes (i.e., in each of an nMISFET and a pMISFET), the mobility of carriers is improved due to the piezo resistivity effect.

Note that in the structure shown in FIGS. 9A and 9B, the two first-type internal stress films 16 cover parts of the side and upper surfaces of the gate electrode 15 located at respective end portions of the gate electrode 15. However, even if the two first-type internal stress films 16 cover only the side surface of the gate electrode 15, a tensile stress in the gate width direction in the channel region 17 x can be generated.

Furthermore, even in the case where the first-type internal stress film 16 is provided only at one end of the gate electrode 15, a tensile stress at a certain level can be generated.

Moreover, in addition to this structure, another first-type internal stress film may be provided under each of end portions of the gate electrode 15.

As has been described above, according to the present invention, a stress is generated in a semiconductor device by an internal stress film, so that a stress can be generated in an arbitrary location on the semiconductor substrate using semiconductor process steps which show affinity to present process steps for fabricating a semiconductor device, thus resulting in increase in the mobility of carriers. 

1. A semiconductor device, comprising a MISFET, wherein the MISFET includes: an active region made of a semiconductor substrate; a gate insulating film formed on the active region; a gate electrode formed on the gate insulating film; source/drain regions formed in regions of the active region located on both sides of the gate electrode; and a silicon nitride film formed over from side surfaces of the gate electrode to upper surfaces of the source/drain regions, wherein: the silicon nitride film is not formed on an upper surface of the gate electrode, and the gate electrode protrudes upward from a surface level of parts of the silicon nitride film located at both side surfaces of the gate electrode.
 2. The semiconductor device of claim 1, wherein the silicon nitride film is for generating a stress in a substantially parallel direction to the gate length direction in a channel region located in the active region under the gate electrode.
 3. The semiconductor device of claim 2, wherein the substantially parallel direction of the stress includes a direction tilted by an angle of less than 10 degree from a direction in which carriers move.
 4. The semiconductor device of claim 1, wherein the silicon nitride film is directly in contact with the source/drain regions.
 5. The semiconductor device of claim 1, wherein the silicon nitride film is formed above the source/drain regions with a thin film interposed therebetween.
 6. The semiconductor device of claim 1, wherein the source/drain regions include lightly doped impurity regions formed in regions of the active region located on both sides of the gate electrode, heavily doped impurity regions formed in regions of the active region respectively extending outwardly from the lightly doped impurity regions to be in contact with the lightly doped impurity regions and having a higher impurity concentration than that of the lightly doped impurity regions, and a silicide layer.
 7. The semiconductor device of claim 1, further comprising: a sidewall formed on the side surface of the gate electrode, wherein the silicon nitride film is formed over the side surfaces of the gate electrode with the sidewall interposed between the silicon nitride film and the side surface of the gate electrode.
 8. The semiconductor device of claim 1, wherein a principal surface of the semiconductor substrate is substantially a {100} plane, and the gate length direction of the gate electrode is substantially a <011> direction.
 9. The semiconductor device of claim 1, further comprising: an interlevel insulating film formed on the silicon nitride film; and a contact plug provided so as to pass through the interlevel insulating film and the silicon nitride film and to be connected to the source/drain regions.
 10. The semiconductor device of claim 1, wherein: the active region is divided by an isolation region formed in the semiconductor substrate, and the silicon nitride film is formed to extend over the isolation region as well as the source/drain regions.
 11. The semiconductor device of claim 10, wherein a lower surface of the isolation region is located in the semiconductor substrate and is in direct contact with the semiconductor substrate.
 12. The semiconductor device of claim 1, wherein the gate insulating film is a silicon oxide film.
 13. The semiconductor device of claim 1, wherein the gate insulating film is a silicon oxynitride film.
 14. The semiconductor device of claim 1, wherein the gate electrode has a polysilicon film.
 15. The semiconductor device of claim 1, wherein the gate electrode has a metal film.
 16. The semiconductor device of claim 1, wherein the silicon nitride film is provided so as to cover at least part of at least one of the source/drain regions.
 17. The semiconductor device of claim 1, wherein the silicon nitride film covers at least respective parts of the source/drain regions.
 18. The semiconductor device of claim 1, wherein the silicon nitride film covers at least respective parts of both side surfaces of the gate electrode.
 19. The semiconductor device of claim 1, wherein the MISFET is an nMISFET and the source/drain regions are n-type source/drain regions.
 20. The semiconductor device of claim 19, wherein the silicon nitride film is for generating a tensile stress in a substantially parallel direction to the gate length direction in a channel region located in the active region under the gate electrode.
 21. The semiconductor device of claim 19, wherein the n-type source/drain regions include n-type lightly doped impurity regions, n-type heavily doped impurity regions and a silicide layer.
 22. The semiconductor device of claim 1, wherein the silicon nitride film directly contacts with the side surfaces of the gate electrode.
 23. The semiconductor device of claim 1, wherein an upper surface of the gate electrode is higher than an upper surface of the parts of the silicon nitride film located at both side surfaces of the gate electrode.
 24. The semiconductor device of claim 1, wherein the gate insulating film is formed only under a lower surface of the gate electrode.
 25. The semiconductor device of claim 1, further comprising: a sidewall formed on the side surface of the gate electrode; an interlevel insulating film formed on the silicon nitride film; an isolation region formed in the semiconductor substrate to divide the active region; and a contact plug provided so as to pass through the interlevel insulating film and the silicon nitride film and to be connected to the source/drain regions, wherein the silicon nitride film is formed over the side surfaces of the gate electrode with the sidewall interposed between the silicon nitride film and the side surface of the gate electrode and to extend over the isolation region as well as the source/drain regions. 